Workpieces to be coated sequentially, for example the unprocessed bodies of motor-vehicles, are moved by either floor mounted or suspended conveying devices through a spraying booth containing a painter-robot, or like spraying device, controlled by a computer having a stored processing and motion program. As is known, the motion program may be set up by a "teach-in" method involving the determination of a plurality of paint-impact points, or locations, on the workpiece body which the painter-robot passes consecutively when the coating unit is in operation. It is desirable for the coating process to be carried out as the workpiece body moves continuously through the spraying booth. Thus, the spraying device of the robot moves not only relative to the workpiece but also in parallel with the movement of the conveying device.
In coating processes of this kind, there is a possibility of being interrupted automatically or manually, for example in the event of a power-failure, in an emergency, or for some other reason, whereupon the robot-painter and the conveying device are halted. In such an event it has hitherto been impossible to resume the coating process by carrying on with the program, since both the conveying device and the painter-robot come to a halt in non-specific terminal positions because of their inertia. Therefore, the resulting new relative positions between the spraying device and the workpiece body are no longer specifically related to the interrupted program, i.e., they are no longer synhronized.
Even if the spraying device were to be moved manually back to the approximate relative position where the coating process was interrupted, automatic continuation of the coating process would still be impossible since the last specific paint-impact point on the workpiece body reached by the program control is unknown. For these reasons, partly coated workpiece bodies have hitherto not been finished but have been removed as rejects. Further, a coating process interruption affects not only one, but several workpiece bodies being coated simultaneously, and thus renders them all rejects.